Charged particle beam systems are widely spread in the semiconductor industry. Examples of charged particle beam devices are electron microscopes such as secondary electron microscopes (SEM), electron beam pattern generators, ion microscopes as well as ion beam pattern generators. Charged particle beams, in particular electron beams, offer superior spatial resolution compared to photon beams, due to their short wavelengths at comparable particle energy.
Charged particle beam systems are used for quality evaluations and control of semiconductor processes, e.g. for inspection of critical dimensions (CD), defect review (DR), or wafer and mask defect inspection. For these tools special low voltage electron optical columns have been developed, in particular SEM-based devices, which either deliver high resolution having electron probes with nanometer and sub-nanometer diameters for CD & DR, or generate high current density electron probes for wafer & mask inspection or hot spot inspection. Because of the high requirements regarding probe size and/or probe current density the focus in the electron optics development was on the optimization of the primary electron beam (PE) optics. Efforts have been made to reduce/optimize the lens aberrations (mainly spherical and chromatic aberration) as well as electron-electron interaction. Recently also some focus was directed on the SE detection, since optimized detection will improve not only the contrast which eases image analysis but also reduces image acquisition time. The latter improves throughput which is of high importance in particular in electron beam inspection (EBI) and HS applications.
In this context, separation of the PE beam and the signal electron (SE) beam, including secondary electrons and backscattered electrons, has been introduced which enables an off-axis detection. Such off-axis detection allows an optimization of the detector independently of constraints imposed by the PE beam. The related SE beam path may be generated by a beam separator for separating the SE beam from the PE beam, followed by a beam bender to increase the SE beam separation from the PE beam. A large separation eases the SE beam detection, in particular by generating space for the SE detector and/or additional SE optics.
The beam bender and SE optics are at a fixed spatial positions, e.g., located on a fixed axis. The PE optics is optimized and aligned for constant column energy, and the excitation of beam shaping elements is fixed. Then, SE beam alignment with the bender and the subsequent SE optics is provided for the chosen operation condition.
In order to choose a different imaging mode, one may change the landing energy of the PE beam by changing the sample bias. However, if the landing energy of the PE beam is changed in this way, the SE beam energy will also change and the SE beam separation angle will vary. In addition, secondary electrons and backscattered electrons will have different separation angles due to their different energies. As a consequence, the SE beam will pass off-axis through the beam bender and subsequent SE optics. Such deviations lead to aberrations in the SE beam path and will disturb the imaging quality of the SE beam.
Consequently, there is a need for an improved charged particle beam device and method of operating the same.